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| United States Patent |
5,578,150
|
|
Suzuki
, et al.
|
November 26, 1996
|
Heat treatment process for wire rods
Abstract
A conveyor moves forward an unconcentrically spiralled loose coil of steel
wire rod having a temperature not lower than Ar.sub.3 into a retention
bath of molten salt for heat treatment. Just before entering the retention
bath, the coil is quenched by spraying a solution of molten salt kept at a
temperature between 400.degree. and 600.degree. C. and not higher than the
temperature of the retention bath either from above and below or from only
above the coil. Then, the quenched coil is retained in the retention bath
of molten salt kept at a temperature between 400.degree. and 600.degree.
C., thereby causing pearlite transformation and forming a fine pearlite
structure in the wire rod.
| Inventors:
|
Suzuki; Takefumi (Futtsu, JP);
Ozaki; Shigekatsu (Futtsu, JP)
|
| Assignee:
|
Nippon Steel Corporation (Tokyo, JP)
|
| Appl. No.:
|
466964 |
| Filed:
|
June 6, 1995 |
Foreign Application Priority Data
| Current U.S. Class: |
148/595; 148/596; 148/600 |
| Intern'l Class: |
E21D 001/00; C21D 001/607 |
| Field of Search: |
148/595,596,600
266/106,112,113
|
References Cited
U.S. Patent Documents
| 3340109 | Sep., 1967 | Keough | 148/153.
|
| 3615926 | Oct., 1971 | Taylor | 148/143.
|
| 3785878 | Jan., 1974 | Economopoulos | 148/596.
|
| 4314860 | Feb., 1982 | Tominaga | 148/596.
|
| Foreign Patent Documents |
| 2015665 | Oct., 1971 | DE | 148/596.
|
| 2435831 | Feb., 1976 | DE.
| |
| 4035155 | Sep., 1991 | DE.
| |
| 56-38426 | Apr., 1981 | JP.
| |
| 56-102524 | Aug., 1981 | JP.
| |
| 63-105933 | May., 1988 | JP.
| |
| 2062692 | May., 1981 | GB.
| |
Primary Examiner: Ip; Sikyin
Attorney, Agent or Firm: Wenderoth, Lind & Ponack
Parent Case Text
This application is a continuation of now abandoned Ser. No. 08/095,207
filed Jul. 23, 1993, abandoned.
Claims
What is claimed is:
1. A heat treatment process for steel wire rod comprising the steps of:
forming an unconcentrically spiralled loose coil of steel wire rod just
rolled and having a temperature not lower than Ar.sub.3 on a conveyor by
means of a take-up reel;
spraying the wire rod being conveyed forward in the unconcentrically
spiralled loose coil with a solution of molten salt at a temperature
between 400.degree. and 600.degree. C. and not higher than a temperature
of a bath of molten salt for retention either from above and below or from
only above, to quench the wire rod to a temperature between 400.degree.
and 600.degree. C.; and
subsequently introducing and retaining the quenched wire rod, in the
unconcentrically spiralled loose coil, in said retention bath of molten
salt kept at a temperature between 400.degree. and 600.degree. C., thereby
producing a fine pearlite structure in the wire rod through pearlite
transformation.
2. A heat treatment process for steel wire rod according to claim 1, in
which the temperature difference between the sprayed solution of molten
salt and the retention bath of molten salt is kept within 40.degree. C.
3. A heat treatment process for steel wire rod according to claim 1, in
which a cooled solution of molten salt is sprayed onto the coil of steel
wire rod.
4. A heat treatment process for steel wire rod according to claim 1, in
which the coil of steel wire rod held in a bath of molten salt is moved
forward by means of a conveyor.
5. A heat treatment process for steel wire rod comprising the steps of:
forming an unconcentrically spiralled loose coil of steel wire rod just
rolled and having a temperature not lower than Ar.sub.3 on a conveyor by
means of a take-up reel;
introducing the coil of wire rod into a bath of molten salt for retention
and spraying the wire rod immediately after the introduction into the bath
with a solution of molten salt at 400.degree. and 600.degree. C. either
from above and below or from only above the unconcentrically spiralled
loose coil in the bath, to quench the wire rod to a temperature between
400.degree. and 600.degree. C.; and
subsequently retaining the quenched wire rod, in the unconcentrically
spiralled loose coil, in said retention bath of molten salt kept at a
temperature between 400.degree. and 600.degree. C., thereby producing a
fine pearlite structure in the wire rod through pearlite transformation.
6. A heat treatment process for steel wire rod according to claim 5, in
which the temperature difference between the sprayed solution of molten
salt and the retention bath of molten salt is kept within 40.degree. C.
7. A heat treatment process for steel wire rod according to claim 5, in
which a cooled solution of molten salt is sprayed onto the coil of steel
wire rod.
8. A heat treatment process for steel wire rod according to claim 5, in
which the coil of steel wire rod held in a bath of molten salt is moved
forward by means of a conveyor.
Description
BACKGROUND
This invention relates to a heat treatment process for steel wire rods,
and, more particularly, to a process for directly heat treating steel wire
rods by utilizing the heat produced during the finish rolling of steel
wire rods.
Heat treatment is necessary to impart high strength and toughness to
hot-rolled hard-steel wire rods. Lead patenting is a common heat treatment
process conventionally employed in the production of high-strength rods.
In addition to this, simpler direct heat treatment processes utilizing the
sensible heat (800.degree. to 1000.degree. C.) conserved in hot-rolled
rods have been developed. For example, Japanese Provisional Patent
Publications Nos. 38426 of 1981 and 102524 of 1981 proposed processes to
dip as-rolled rods directly in a solution of salt.
A process disclosed in Japanese Provisional Patent Publication No. 38426 of
1981 uses a low-temperature bath of molten salt 14 and a high-temperature
bath of molten salt 15, as shown in FIG. 3. A coil of steel wire rod 1
falling onto a roller conveyor 3 from a laying head 2 of a take-up reel
moves forward in an unconcentric spiral. The moving rod is first cooled in
the low-temperature bath of molten salt 14 in which a sorbite structure is
formed and then in the high-temperature bath of molten salt 15 where
untransformed austenite is completely transformed into sorbite. This
process requires both a low-temperature bath for quenching and a
high-temperature bath for retention, as one bath cannot provide adequate
cooling.
Another process disclosed in Japanese Provisional Patent Publication No.
102524 of 1981 obtains a fine structure of pearlite by uniformly cooling
wire rods in a bath of molten salt whose rate of heat transfer is
increased by agitating with air or other gases satisfying specific
requirements.
Though the processes employing two or one bath of molten salt just
described produce wire rods having high strength and toughness comparable
to those obtained by lead patenting, they involve the following new
problems.
In the former process, undercooling of the surface and subsurface area to a
temperature considerably lower than that desirable for transformation
(which is substantially equal to the temperature of the high-temperature
bath for retention) produces bainite. The use of two baths, one for
quenching (at approximately 400.degree. C.) and one for retention (at
approximately 550.degree. C.), necessitates troublesome temperature
control of the individual baths as well as higher equipment investment and
running cost. The latter process also gives rise to a problem of bainite
formation resulting from the undercooling of the surface.
SUMMARY
The object of this invention is to provide a heat treatment process to
produce steel wire rods having high strength and toughness at low cost
with one bath of molten salt while solving the problems encountered by the
conventional processes as described before.
A heat treatment process for steel wire rods to achieve the above object of
this invention comprises the steps of forming an unconcentrically
spiralled loose coil of steel wire rod just rolled and having a
temperature not lower than Ar.sub.3 on a conveyor by means of a take-up
reel, quenching the wire rod being conveyed forward by spraying a solution
of molten salt at a temperature between 400.degree. and 600.degree. C. and
not higher than the temperature of a bath of molten salt for retention
either from above and below or from only above and subsequently retaining
the quenched wire rod in said retention bath of molten salt kept at a
temperature between 400.degree. and 600.degree. C., thereby producing a
fine pearlite structure through pearlite transformation.
Another heat treatment process of this invention comprises the steps of
introducing a coil of wire rod into a bath of molten salt and quenching
the wire rod immediately after the introduction into the bath by spraying
a solution of molten salt at 400.degree. and 600.degree. C. either from
above and below or from only above the coil in the bath.
In the above processes, a cooled solution of molten salt may be sprayed to
the coil of wire rod, or retention may be effected by conveying the coil
of wire rod placed in a bath of molten salt.
Spraying a solution of molten salt onto the coil of wire rod assures a high
cooling efficiency and permits attaining the desired goal with only one
bath of molten salt.
DRAWINGS
FIG. 1 is a overall schematic view of an apparatus for implementing a
process of this invention.
FIG. 2 is an overall schematic view of another apparatus for implementing a
process of this invention.
FIG. 3 is an overall schematic view of an apparatus for implementing a
conventional dual-salt-bath process.
FIG. 4 is a graphical representation of a TTT curve and a cooling curve.
FIG. 5 graphically compares the relationships of the surface temperature to
the heat transfer coefficient in a process of this invention and a process
tested for the purpose of comparison.
DESCRIPTION
To obtain wire rods having a fine pearlite structure, pearlite
transformation must be caused by quenching the wire rod from near
1000.degree. C. and retaining the quenched rod at a given temperature. For
example, the wire rod must be quenched to a temperature at the nose of the
TTT curve shown in FIG. 4 and then retained at a given temperature
(usually approximately 550.degree. C.). If only one bath is used in which
the temperature is kept at the quenching temperature that is lower than
the retention temperature, the desired retention temperature cannot be
maintained as a result of undercooling. Conversely, quenching is
impractical if the bath temperature is kept at the retention temperature.
This is the reason why two baths have conventionally been employed to
carry out quenching and retention separately.
This invention has obviated the above difficulty by quenching the hot wire
rod fresh from the rolling process by spraying a solution of molten salt
either above or in the entry end of a bath of molten salt, thereby
increasing the heat transfer coefficient of the quenched part of the rod
by a factor of two to three over the conventional level. The heat flux in
the cooled steel is proportional to h.times..DELTA.T (where h= heat
transfer coefficient and .DELTA.T= temperature difference between the
cooling medium and the surface of the cooled steel). When the rod
temperature is high, accordingly, .DELTA.T is large and the cooling rate
is high. If the rod temperature drops, however, both .DELTA.T and the
cooling rate decrease. The spray of a solution of molten salt employed in
the process of this invention maintains a high heat transfer coefficient
in the wire rod even when its temperature drops, as indicated by curve A
in FIG. 5. The two to three times higher heat transfer coefficient than
conventional thus obtained permits maintaining a high cooling rate even
when the rod temperature drops. In FIG. 5, curves B and C show the heat
transfer coefficients in the conventional dip and gas-agitation processes.
Details of a heat treatment process of this invention employing a single
bath of molten salt will be described by reference to the accompanying
drawings.
FIG. 1 shows an apparatus for implementing a heat treatment process of this
invention. Reference numeral 1 designates wire rod, 2 a laying head, 3 a
roller conveyor, and 4 a bath of molten salt 4a into which the wire rod 1
is dipped. In this apparatus, the top surface of the roller conveyor 3 is
kept above the surface of the bath 4 over a given distance from the entry
end thereof. In this elevated region, the wire rod 1 on the conveyor 3 is
forcibly cooled by a solution of molten salt 8a, 8b sprayed from above and
below (or only from above). This spray system comprises a series of top
nozzles 7a and bottom nozzles 7b disposed in the direction of rod travel,
with a molten salt pump 5, a top nozzle header 6a and a bottom nozzle
header 6b connected thereto. Reference numeral 9 denotes a molten salt
cooler interposed between the pump 5 and bath 4 to suck the warmed
solution of molten salt 4a from the bath 4, cool the solution back to the
predetermined bath temperature and return the cooled solution to the
nozzle headers 6a, 6b.
In this apparatus, the wire rod 1 falling onto the roller conveyor 3 from
the laying head 2 of a take-up reel moves forward in a loose
unconcentrically spiraled coil 1a. On entering the space above the salt
bath 4, but not in the bath 4 itself yet, the wire rod 1 on the conveyor 3
over a given distance from the entry end thereof is quenched by a solution
of molten salt 8a, 8b directly sprayed from the nozzles 7a and 7b above
and below. The wire rod 1 thus quenched then enters the salt bath 4 itself
for retention and then leaves the bath after a given period of time to
continue its travel into the following process.
FIG. 2 shows another apparatus to implement the heat treatment process of
this invention, in which molten salt spraying is applied in the salt bath
4. Unlike the apparatus shown in FIG. 1, a solution of molten salt 8a, 8b
is sprayed from above and below the wire rod not outside but inside the
bath 4 of molten salt 4a. Therefore, the top surfaces of the rollers of
the conveyor 3 are kept below the bath surface throughout the entire
length of the bath 4. Like reference characters denote parts similar to
those in FIG. 1.
The cooling operation and function of the apparatus shown in FIG. 2 are
essentially similar to those of the apparatus shown in FIG. 1, with the
exception of a few minor differences. For example, the apparatus in FIG. 2
dispenses with the need for means to be taken against the mist resulting
from spraying. On the other hand, the tip of the nozzles disposed inside
the bath must be brought closer (not more than approximately 300 mm away)
to the wire rod.
The wire rod delivered to the heat treatment process of this invention,
whether on the apparatus shown in FIG. 1 or the one in FIG. 2, has been
finish-rolled at a temperature at least not lower than Ar.sub.3 (usually,
finish-rolled hard-steel wire rod has a sensible heat of 800.degree. to
1000.degree. C.). To obtain a fine pearlite structure, such as rolled wire
rod must be quenched to a temperature between 400.degree. and 600.degree.
C. and, then, retained in the same temperature range in a bath of molten
salt. The temperature of the salt bath is kept either equal to the lower
limit of the pearlite transformation temperature which, though it varies
with the composition of steel, is approximately 500.degree. to 600.degree.
C. or in a lower range of 400.degree. to 600 .degree. C. The solution of
molten salt sprayed is kept between 400.degree. and 600.degree. C. and not
higher than the above temperature of the retention salt bath. The
temperature difference between the salt spray and retention bath should
preferably be kept within 40.degree. C. because undercooling results if
the temperature of the salt spray is much lower than that of the retention
bath.
With the heat treating temperature ranges thus preset, pearlite
transformation begins in the wire rod quenched in the bath of molten salt
in which the quenched rod is subsequently retained for a given period of
time until pearlite transformation is complete, whereupon a fine pearlite
structure is formed in the wire rod.
The salt spraying devices should not be limited to those shown in FIGS. 1
and 2. Other conventional spraying devices may also be used if they
function similarly. Also, the travel of the wire rod in the salt bath may
be suspended for a given period of time to achieve the desired retention.
Now an example of wire rod heat treated by the process of this invention is
described below, together with two examples of wire rod heat treated by a
conventional process involving gas agitation.
The specimens were taken from wire rods having a diameter of 8 mm and a
chemical composition shown in Table 1. The specimens were heat treated so
that transformation occurs at temperatures near the targeted temperature
of 552.degree. C. at any point of the cross section. The molten salt used
in the heat treatment consisted of 50% of NaNO.sub.3 and 50% KNO.sub.3.
Table 2 shows the cooling conditions employed. Conventional process 1
tested for comparison is the most effective one among the conventional
processes. To achieve rapid cooling during the initial stage, the
temperature of the first bath was kept considerably lower than the
targeted transformation temperature. Conventional process 2 for
comparison, like conventional process 1 for comparison, also employed
cooling with agitation. However, process 2 for comparison used only one
bath whose temperature was kept substantially equal to the targeted
transformation temperature to prevent the undercooling of the surface of
the specimen.
Table 3 shows the mean transformation temperatures at different selected
points in the cross section of the specimens. In the surface of the
specimen heat treated by conventional process 1 for comparison,
transformation took place at a temperature lower than the targeted
temperature because of the low temperature of the first bath, thus
producing supercooled bainite in that part. On the other hand, the
temperatures at the different selected points of the specimen heat treated
by conventional process 2 did not reach the targeted transformation
temperature while scattering considerably because of inadequate cooling.
By contrast, the specimen heat treated by the process of this invention
exhibited a uniformly transformed structure, with the temperatures at the
different selected points therein varying little from each other and
differing little from the targeted transformation temperature.
TABLE 1
______________________________________
Chemical Composition of Specimens
C Si Mn P S Al
______________________________________
0.842%
0.236% 0.76% 0.013% 0.007%
0.025%
______________________________________
TABLE 2
______________________________________
Cooling Conditions
Cooling Salt Bath Tem-
Starting perature (.degree.C.)
Tempera- Bath Bath Spraying
Process
ture (.degree.C.)
No. 1 No. 2 Conditions
______________________________________
Process
850 550 -- Flow density of
of This salt spray:
Invention 2000 l/m.sup.2 .multidot. min.
(One
Bath)
Process 1
850 480 550 Agitated with
for Com- 500 l/m.sup.2 .multidot. min. of
parison, gas
with Gas
Agitation
(Two
Baths)
Process 2
850 550 -- Agitated with
for Com- 500 l/m.sup.2 .multidot. min. of
parison, gas
with Gas
Agitation
(One
Bath)
______________________________________
TABLE 3
______________________________________
Transformation Temperatures
Distance from Surface (mm)
Process 0.1 1.0 2.0 4.0
______________________________________
Process 550 551 552 554
of This
Invention
Process 1 520 534 542 555
for Com-
parison,
with Gas
Agitation
(Two
Baths)
Process 2 612 623 634 642
for Com-
parison,
with Gas
Agitation
(One
Bath)
______________________________________
As described above, the heat treatment process of this invention provides
adequate cooling with one quenching bath. As such, the process of this
invention can be used to great advantage in the production of high-quality
steel wire rods with much less equipment investment and running cost.
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